The present invention relates to a copper foil for a printed wiring board having excellent chemical resistance and adhesiveness, a producing method thereof, a resin substrate for a printed wiring board and a printed wiring board. In particular, the present invention provides a copper foil and a producing method thereof as well as a printed wiring board, wherein strong peel strength against chemical treatment when forming a fine pattern on a package board such as a BT (bismaleimide triazine) resin impregnated substrate can be obtained to allow fine etching. Further, the present invention provides a copper foil for a printed wiring board, a producing method thereof, a resin substrate for a printed wiring board and a printed wiring board, wherein peel strength can be improved significantly in a method of forming a copper pattern by non-electrolytic plating after complete etching of a surface of the copper foil.
A copper foil for a semiconductor package board, which is also called a copper foil for a printed wiring board in general, is usually fabricated by the following steps. First, a copper foil is laminate-bonded to a substrate such as a synthetic resin under high temperature and high pressure. Subsequently, in order to form a conductive circuit of interest on the substrate, a circuit equivalent to that circuit is printed on the copper foil using a material such as an etching resistant resin.
Then an unwanted and exposed region of the copper foil is removed by etching treatment. After etching, the printed region of the material such as resin is removed to form a conductive circuit on the substrate. Finally, a predetermined element is soldered to the conductive circuit formed to make various printed circuit boards for electronics devices. Eventually, it is bonded to a resist or a build-up resin substrate. In general, quality requirements for a copper foil for a printed wiring board are different between an adhesive surface to be bonded to a resin substrate (a so-called roughened surface) and a non-adhesive surface (so-called a glossy surface), both of which are required to be satisfied at the same time.
Requirements for a glossy surface include: (1) good appearance and no oxidative discoloration during storage; (2) good solder wettability; (3) no oxidative discoloration upon high temperature heating; (4) good adherence with a resist. On the other hand, requirements for a roughened surface mainly include: (1) no oxidative discoloration during storage; (2) sufficient peel strength maintained after high temperature heating, wet processing, soldering, chemical treatment and the like; (3) no so-called lamination staining generated after lamination with a substrate and etching.
In addition, a low profiled copper foil is increasingly demanded as a pattern becomes finer in recent years. Thus, increased peel strength is needed for a roughened surface of a copper foil.
Further, electronic devices such as personal computers and mobile communications require a printed wiring board and a copper foil which can handle electric signals of increasingly higher frequency according to increased speed and capacity of communications. When a frequency of an electric signal is 1 GHz or higher, the skin effect, in which an electric current flows only on the surface of a conductor, becomes significant, and an uneven surface causes a change in electric current transmission paths, resulting in a non-negligible increase in impedance. In view of this, reduced surface roughness of a copper foil is also desirable. In order to meet these demands, a large number of methods for treating a copper foil for a printed wiring board have been proposed.
In the methods for treating a copper foil for a printed wiring board, in general, a rolled copper foil or an electrolytic copper foil is used; and first, roughening treatment is performed where particles generally comprising copper and copper oxide are attached to the surface of the copper foil in order to increase adhesiveness (peel strength) between the copper foil and a resin. Subsequently, in order to give properties of heat resistance and rust proofing, a heat resistant layer, in another word, ‘a barrier layer’ such as brass or zinc is formed. Rust proof treatment such as immersion or electrolytic chromate treatment or electrolytic chromium and zinc treatment is performed thereon for prevention of surface oxidation and so on during transportation or storage. Thus, a product is obtained.
Among these, the roughened layer particularly plays a significant role on increasing the adhesiveness (peel strength) between the copper foil and the resin. Conventionally, roundish or spherical projections have been thought to be good for this roughening treatment. The roundish projections are to be achieved by suppressing a growth of dendrites. However, the roundish projections are detached at the time of etching, causing a phenomenon called “powder fall.” This phenomenon is not surprising because the contact area between a spherical projection and a copper foil is much smaller compared to the diameter of the roundish or spherical projection.
In order to avoid this phenomenon of “powder fall,” a thin copper plating layer is forming over the projections after the above-mentioned roughening treatment to prevent the detachment of the projections (see Patent Literature 1). Although this has an effect of preventing “powder fall,” a problem is that the number of steps is increased, and that the effects of preventing “powder fall” vary because the copper plating is thin.
In addition, a technology is known in which a needle-like nodular coating layer comprising an alloy of copper and nickel is formed on a copper foil (Patent Literature 2). Since the nodular coating layer is needle-like, the adhesiveness with a resin is thought to be increased compared to the roundish or spherical projection disclosed in Patent Literature 1. On the other hand, the layer, which is made of a copper-nickel alloy, has different components from the underlying copper foil, and has a different etching speed when a copper circuit is formed by etching. Therefore, there exists a problem that it is unsuitable for a stable circuitry design.
When forming a copper foil for a printed wiring board, a heat resistant and rust proof layer is generally formed. A large number of copper foils over which a coating layer is formed such as Zn, Cu—Ni, Cu—Co and Cu—Zn as examples of metals or alloys to form a heat resistant layer are in practical use (for example, see Patent Literature 3).
Among these, a copper foil over which a heat resistant layer of Cu—Zn (brass) is formed is widely used in industry because it shows no stain in a resin layer when layered over a printed circuit board of an epoxy resin and the like, and because it has excellent properties such as less deteriorated peel strength after high temperature heating.
A method of forming this heat resistant layer of brass is described in detail in Patent Literature 4 and Patent Literature 5.
A copper foil over which such a heat resistant layer of brass is formed is then subject to etching treatment to form a printed circuit. Recently, a hydrochloric acid based etching solution is becoming more popular for use in forming a printed circuit.
However, when a printed circuit board over which a heat resistant layer of brass is formed was subject to etching treatment with a hydrochloric acid based etching solution (for example, CuCl2, FeCl3), and an unwanted region of the copper foil except for a region of the printing circuit was removed to form a conductive circuit, a phenomenon of so-called erosion (circuit erosion) occurred at the both ends of the circuit pattern, causing a problem that the peel strength with the resin substrate was deteriorated.
The phenomenon of circuit erosion is referred to as a phenomenon where erosion occurs by the aforementioned etching solution at the adhesion interface layer between a copper foil of a circuit formed by the aforementioned etching treatment and a resin substrate, i.e. at the side of etching where the heat resistant and rust proof layer of brass is exposed, and the both ends, which are otherwise normally yellow (because it is made of brass), become red due to the erosion because water washing is insufficient, resulting in significantly deteriorated peel strength in those regions. Thus, if this phenomenon occurs over an entire circuit pattern, the circuit pattern will be detached from the substrate, causing a problem.
In view of these, an attempt has been proposed where after performing roughening treatment, rust proof treatment with zinc or a zinc alloy and chromate treatment on a surface of a copper foil, a silane coupling agent containing a small amount of chromium ions is allowed to be adsorbed in the surface after the chromate treatment to improve its resistance to hydrochloric acid (see Patent Literature 7).    Patent Literature 1: Japanese Patent Laid-Open Publication No. H08-236930    Patent Literature 2: Japanese Patent No. 3459964    Patent Literature 3: Japanese Examined Patent Publication No. S51-35711    Patent Literature 4: Japanese Examined Patent Publication No. S54-6701    Patent Literature 5: Japanese Patent No. 3306404    Patent Literature 6: Japanese Patent Application No, 2002-170827    Patent Literature 7: Japanese Patent Laid-Open No. H03-122298